The invention relates to the determination of noble metal concentrations in either a volume of water containing such noble metals or in components exposed to such water. More particularly, the present invention relates to a system and method for determining noble metal concentrations in either a volume of water or in components exposed to such water. Even more particularly, the invention relates to a system and method for determining the concentration of noble metals present in either nuclear reactor water during noble metal chemical addition to the water or in the surface of structural materials that have been exposed to the reactor water containing the noble metals.
Under the water chemistry conditions normally encountered during the operation of boiling water nuclear reactors (BWRs), strong oxidizing species, such as oxygen and hydrogen peroxide, are generated. The presence of such oxidizing species contribute to the intergranular stress corrosion cracking (IGSCC) of sensitized 304 stainless steel within the reactor. Naturally, IGSCC is known to be a major environment-related material performance problem within BWRs. It has been demonstrated that, by sufficiently lowering the concentrations of ionic impurities and oxidizing species in the reactor water, IGSCC can be mitigated. The electrochemical corrosion potential (ECP) of stainless steels and other active metals is known to be controlled mainly by the dissolved oxygen, hydrogen, and hydrogen peroxide concentrations in the BWR coolants and the hydrodynamic flow conditions within the coolant path. In order to evaluate or predict materials performance (including SCC as a function of time), it is extremely important to know the ECP value of the structural materials that are exposed to high temperature water within the reactor pressure vessel.
In hydrogen water chemistry (HWC), hydrogen is added to the feed water of a BWR to mitigate IGSCC. The primary purpose of the hydrogen addition is to reduce the concentrations of dissolved oxidants and thus lower the ECP to a value that is less than a critical value of xe2x88x92230 mV, measured against a standard hydrogen electrode (SHE), at which IGSCC susceptibility is markedly reduced. Hydrogen (H2) levels in the feed water are always in the excess of the stoichiometric amount needed to react with either O2 or H2O2 to form H2O. However, several side effects of the HWC application, such as increased N16 carry-over to the turbine and higher Co60 deposition rates, have been reported. Also, the critical ECP value that is needed to prevent IGSCC is difficult to achieve in highly oxidizing and/or high water flow regimes.
Subsequent to the development of HWC, noble metal technology (NMT) was developed. By improving the catalytic properties of metal surfaces for the recombination of either hydrogen/oxygen or hydrogen peroxide/hydrogen to form water, NMT allows low ECP values to be achieved at much lower H2 addition rates. This catalysis reduces the oxygen concentration at the metal surface to zero, thus causing the ECP to drop to its thermodynamic minimum (about≈xe2x88x92550 mVSHE in pure water at 288xc2x0 C.). To achieve a stoichiometric excess of hydrogen, a H:O molar ratio of greater than 2:1, or a H:O weight ratio of greater than 1:8, is needed. This condition has been demonstrated to occur not only for pure noble metals and coatings, but also for very dilute noble metal alloys (NMA) or thermal spray coatings with powders of NMA. Recently, a technique for in-situ noble metal chemical addition (NMCA) on the oxide surfaces of various structural materials in high temperature water has been developed and applied to commercial BWRs in the United States, Europe, and Japan. Using NMCA, chemicals containing noble metals are injected directly into the reactor water and then are deposited onto the surfaces of reactor components that are exposed to the feed water. The surfaces of the reactor components are typically covered with an oxide outer layer. The noble metals are deposited onto the oxide layer, thus providing a catalytic site for both the H2/O2 and H2/H2O2 recombination reactions. The ECP value needed to ensure protection of components from IGSCC can then be achieved through the addition of smaller amounts of hydrogen, thus avoiding many of the negative side effects that are frequently encountered at higher H2concentrations.
In order to control the loading levels of noble metals such as platinum (Pt) and rhodium (Rh), the NMCA application process that is currently used requires that the concentration of noble metals on both the surface of the reactor components and in the reactor water can be determined. In order to measure the noble metal concentration present on the surface of the reactor components both during and after the NMCA application, the oxide surfaces that have been treated with noble metals (such as Pt and Rh) are first immersed in aqua regia to dissolve the oxide layer containing the noble metals. A sample taken from the aqua regia solution is introduced into an inductively coupled plasma-mass spectrometer (ICP-MS) to determine the noble metal concentration. Because of the relatively long time required to dissolve the oxide layer containing the noble metals in aqua regia, about 3-4 hours are needed to obtain valuable information on the Pt and Rh concentrations by this analytical method. As the NMCA process normally takes about 48 hours to deposit the desired amount of noble metal on the surfaces of BWR components that are exposed to high temperature feed water, the ICP-MS method of analysis is unable to provide a timely determination of the noble metal concentration in either the feed water or the component surface.
In addition to the long period of time needed to dissolve the oxide layer containing the noble metals, the use of ICP-MS to determine the noble metal concentrations during the NMCA process has other disadvantages. One such disadvantage is the high cost of ICP-MS hardware. In addition to cost, an ICP mass spectrometer typically requires a dedicated lab environment, provides no in-situ analytical capability, and requires the use of hazardous reagents such as aqua regia solutions. Furthermore, the sharing of ICP-MS resources by multiple users is precluded by scheduling concerns. All commercial BWRs are treated using the NMCA process during reactor shutdowns and most follow a common regular shutdown schedule. Thus, ICP-MS instruments are in high demand during the periods when such shutdowns take place.
The ICP-MS method of determining the noble metal concentration in BWR feed water and on component surfaces is slow, costly, and logistically awkward. Therefore, what is needed is a cost-effective system for determining the concentration of noble metals in the feed water of a BWR and BWR components that are exposed to the feed water. What is also needed is a timely, cost-effective method for analyzing the noble metal concentration in the feed water of a BWR. Finally, what is also needed is a timely, cost-effective method of determining the noble metal concentration in surfaces of BWR components exposed to feed water containing noble metals in solution.
The present invention meets these and other needs by providing a new system and method for detecting and quantifying the amount of noble metals, such as platinum and rhodium, either dissolved in a volume of water or deposited onto a solid that has been exposed to the volume of water. More particularly, the present invention provides a system and method for determining the noble metal concentration in either the reactor water or the surface of reactor materials that have been exposed to reactor water containing noble metals. The system and method are capable of determining noble metal concentrations during periods of noble metal addition or during plant operation following such addition.
Accordingly, one aspect of the present invention is to provide a system for determining a noble metal concentration in a collection sample, the collection sample having a surface and at least one noble metal disposed thereon. The noble metal concentration in the collection sample is representative of a first concentration of the noble metal in one of a volume of water and a surface of a solid component exposed to the volume of water. The system comprises: at least one standard having a standard surface and a predetermined concentration of the noble metal disposed thereon; an electrolyte bath for immersing one of the collection sample and the standard therein; an auxiliary electrode electrically connectable to one of the collection sample and the standard and being immersible in the electrolyte bath; and a power source electrically connectable to a reference electrode and one of the standard and the collection sample, the reference electrode being immersible in the electrolyte bath; wherein the power source is capable of providing a potential across the reference electrode and one of the collection sample and the standard, and a current measurement device capable of measuring a current passing between the auxiliary electrode and one of the collection ample and the standard. The noble metal concentration in the collection sample is determined relative to the predetermined concentration in the standard by comparing a ample current passing through the collection sample to a standard current passing through the standard.
A second aspect of the invention is to provide a cyclic voltametric apparatus for measuring a current produced by formation of one of hydrogen and oxygen in the presence of at least one noble metal. The cyclic voltametric apparatus comprises: an electrode having a surface and the noble metal disposed thereon, an auxiliary electrode electrically connectable to the electrode and a reference electrode, each of the electrode, the auxiliary electrode, and the reference electrode being immersible in an electrolyte bath; a power source electrically connectable to each of the electrode and the reference electrode, the power source being capable of providing a potential between the reference electrode and the electrode and cyclically varying the potential between at least two predetermined potentials relative to the reference electrode; and a current measurement device capable of measuring a current passing between the electrode and the auxiliary electrode. The hydrogen current produced by formation of hydrogen and an oxygen current produced by formation of oxygen are measured by the current measurement device during at least one reversibly cyclic application of the potential between a first potential at which hydrogen forms and a second potential at which oxygen forms.
A third aspect of the invention is to provide a system for determining a noble metal concentration in a collection sample, the collection sample having a surface and at least one noble metal disposed thereon. The noble metal concentration in the collection sample is representative of a first concentration of the noble metal in one of a volume of water in a boiling water nuclear reactor and a surface of a solid component in the boiling water nuclear reactor that is exposed to the volume of water. The system comprises: at least one standard having a standard surface and a predetermined concentration of the noble metal disposed thereon; an electrolyte bath for immersing one of the collection sample and the standard therein, the electrolyte bath comprising an inorganic acid; an auxiliary electrode, the auxiliary electrode being electrically connectable to one of the collection sample and the standard, and a reference electrode, each of the auxiliary electrode and the reference electrode being immersible in the electrolyte bath; a power source electrically connectable to the reference electrode and one of the collection sample and the standard, the power source being capable of providing a potential between the reference electrode and one of the collection sample and the standard and cyclically varying the potential between at least two predetermined potentials relative to the reference electrode; and a current measurement device capable of measuring a current passing between the auxiliary electrode and one of the collection sample and the standard. A hydrogen current produced by formation of hydrogen in the electrolyte bath and an oxygen current produced by formation of oxygen in the electrolyte bath are measured by the current measurement device during at least one reversibly cyclic variation of the potential between a first potential at which hydrogen forms and a second potential at which oxygen forms. The noble metal concentration in the collection sample is determined relative to the predetermined concentration by comparing a collection sample hydrogen current and a collection sample oxygen current measured for the collection sample to a standard hydrogen current and a standard oxygen current measured for the standard.
A fourth aspect of the present invention is to provide a method for determining a noble metal concentration in a collection sample, the collection sample containing at least one noble metal in a concentration that is representative of a noble metal concentration in one of a volume of water and a surface of a solid component exposed to the volume of water. The method comprises the steps of: immersing the collection sample into an electrolyte solution; connecting the collection sample to an auxiliary electrode; connecting the collection sample and a reference electrode to a power source; applying a potential between the collection sample and the reference electrode; measuring a current passing between the collection sample and the auxiliary electrode; providing at least one standard having a predetermined concentration of the noble metal; immersing the standard into a second electrolyte solution; connecting the collection sample to an auxiliary electrode; connecting the standard and a reference electrode to a power source; applying a potential between the standard and the reference electrode; measuring a current passing between the standard and the auxiliary electrode; and comparing the current passing through the collection sample to the current passing through the standard, thereby determining the concentration of noble metal present in the collection sample relative to the predetermined concentration of noble metal present in the standard.
Finally, a fifth aspect of the present invention is to provide a method of determining a noble metal concentration in a collection sample that is representative of a noble metal concentration in one of a volume of water circulated through a nuclear reactor and a surface of a nuclear reactor component exposed to the volume of water. The method comprises the steps of: providing at least one collection sample, exposing the collection sample to the volume of water; immersing the collection sample into an electrolyte solution; connecting the collection sample to an auxiliary electrode; connecting the collection sample and a reference electrode to a power source; applying a potential between the collection sample and the reference electrode; measuring a current passing between the collection sample and the auxiliary electrode; providing at least one standard having a predetermined concentration of the noble metal; immersing the standard into a second electrolyte solution; connecting the standard to an auxiliary electrode; connecting the standard and a reference electrode to a power source; applying a potential between the standard and the reference electrode; measuring a current passing between the standard and the auxiliary electrode; and comparing the current passing through the collection sample to the current passing through the standard, thereby determining the concentration of noble metal present in the collection sample relative to the predetermined concentration of noble metal present in the standard.
These and other aspects, advantages, and salient features of the invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims.